Incorrect validation

I have 2 fields
"Next Contract Renewal Date" & "Remind me __ days before renewal"

I need some valiation on these fields "Remind me....." already has <=45 to keep the number of days less than 45
So I need to put it on the Renewal Date field.

Here is what I need:
ONLY IF both fields are populated then the calculation of (Date - # of days) should not be in the past i.e. it can be equal to today or any other date in the future.

here is what I have:

([<dNext_Contract_Renewal_Date_ITAG>] IS NOT NULL) AND ([<iContract_Renewal_reminder_date_ITAG>] IS NOT NULL) AND ([<dNext_Contract_Renewal_Date_ITAG>]-[<iContract_Renewal_reminder_date_ITAG>]>= Today())